function fn(x, y) {
    console.log(this.a, x + y);
}
var obj = {
    a: 2,
}
fn.call(1, 2, 3);//undefined 5
fn.call(obj, 2, 3);//2 5
fn.apply(obj, [2, 3]);//2 5
const bar = fn.bind(obj, 2, 3)//优先使用bind里的参数
bar();//2 5s